In theory, just opening is fine, but in practice there are quite a few reports of problems that can crop up late in editing with converted CS3 files and I highly recommend that you export the template to.inx, then open that instead (you can resave as a new CS5 template if you like).

Something else that affects templates in particular is the change in locking behavior. By default a locked item in CS5 cannot be selected. As a side effect this also means that you cannot place content into a placeholder frame that you have locked for position. You can get back the ability to place into a locked frame by changing the behavior in the application preferences.